Members of the small GTPase family, including Ras and Rho-like proteins, function as key cellular relays of signals emanating from cell membrane receptors, by cycling between inactive GDP-bound and active GTP-bound sates. Rho GTPases are regulators of a wide spectrum of cellular functions in eukaryotic cells, including cytoskeletal organization, gene transcription and cell cycle progression. In addition, Rho GTPases have also been shown to play a role in kinase signaling which appear to be parallel if not independent of their roles in actin organization. Rac2-deficient mice have been generated by homologous recombination and manifest defects in neutrophil chemotaxis, superoxide production, shear-dependent L-selectin-mediated capture on endothelial substrates Glycam-1, and F-actin generation. However, it has recently become clear that phenotypic abnormalities of Rac2-/- mice are not confined to the neutrophil lineage, since Rac2-deficient mast cells demonstrate significant actin-based functional defects in vitro, including defective adhesion via integrin receptors and are deficient in vivo in Rac2-/- mice, and Rac proteins also appear critical in lymphocyte differentiation. Indirect evidence from other laboratories suggest that Rac may be involved in Ras hyperactivity characteristic of cells deficient in the Ras GAP protein, neurofibromin, and in the abnormalities associated with expression of the BCR/ABL kinase seen in CM transformed myeloid cells. In this grant application, we propose to utilize a genetic approach to examine the involvement of Rac in signaling in BCR/ABL-induced abnormal cell behavior and NF-1-mediated hyperproliferation. We will utilize genetic crosses between Nf-1 and Rac2 mutant mice to address the role of Rac2 in Ras signaling in primary cells. In addition, we will utilize retrovirus-mediated gene transfer and expression of p210/BC4/ABL in Rac2-deficient myeloid cells to assess the role of Rac2 signaling in BCR/ABL-mediated cell phenotype changes in vitro and development of myeloproliferative disease in vivo. As a result of the proposed studies we will directly determine if Ras proteins are involved in abnormal hematopoiesis characteristics of these two genetic conditions.